Programming mammalian cells with large synthetic gene networks is expected to play a central role in helping elucidate complex regulatory cellular mechanisms (41, 42, 13, 43), implementing new useful biological functions (44, 14, 45) and accelerating the design of novel tailor-made therapeutic treatments (46, 47, 17). Complex, stable and heritable programming of mammalian cells by genomic engineering is limited by the requirement to pre-integrate a natural recombination site at single or multiple genomic (chromosomal) loci, thus necessitating the identification of programmable orthogonal (independently acting) recombinases that can be directly targeted sequentially or simultaneously to the endogenous sequences of choice in the mammalian genome.